Understanding chirality and stereochemistry: three-dimensional psychopharmacology.

Drugs and the places in the body they bind to are each three-dimensional molecular structures. Stereoisomers are compounds that possess the same molecular and structural formula but differ in their three-dimensional configuration. Chiral compounds have two mirror-image stereoisomer forms called enantiomers. Compounds that contain both mirror-image enantiomers in equal proportions are referred to as racemic mixtures or racemates. The presence of a second enantiomer in racemic mixtures can be associated with various problems. The second enantiomer may be less biologically active but could interfere with the more biologically active enantiomer by binding to the same site. Adverse reactions can sometimes be attributed to one of the enantiomers. Each enantiomer may bind to metabolic enzymes differently, resulting in different rates of metabolism and clearance. They may interact differently with other drugs. The investigation, development, and use of stereochemically pure drugs have been encouraged by the U.S. Food and Drug Administration to improve the efficacy, tolerability, and safety of marketed drugs.